Color display device

ABSTRACT

The present invention provides a realistic solution for a highlight or multicolor display device which can display high quality color states. More specifically, an electrophoretic fluid is provided which comprises four types of particles, having different levels of size, threshold voltage or charge intensity.

This application is a continuation of copending application Ser. No.14/989,534, filed Jan. 6, 2016 (Publication No. 2016/0116818), whichitself is a continuation-in-part of application Ser. No. 14/256,768,filed Apr. 18, 2014 (now U.S. Pat. No. 9,285,649, issued Mar. 15, 2016),which claims the benefit of U.S. Provisional Application No. 61/813,551,filed Apr. 18, 2013. The above applications are incorporated herein byreference in their entirety.

FIELD OF THE INVENTION

The present invention is directed to a color display device which candisplay high quality color states, and an electrophoretic fluid for suchan electrophoretic display.

BACKGROUND OF THE INVENTION

In order to achieve a color display, color filters are often used. Themost common approach is to add color filters on top of black/whitesub-pixels of a pixellated display to display the red, green and bluecolors. When a red color is desired, the green and blue sub-pixels areturned to the black state so that the only color displayed is red. Whena blue color is desired, the green and red sub-pixels are turned to theblack state so that the only color displayed is blue. When a green coloris desired, the red and blue sub-pixels are turned to the black state sothat the only color displayed is green. When the black state is desired,all three-sub-pixels are turned to the black state. When the white stateis desired, the three sub-pixels are turned to red, green and blue,respectively, and as a result, a white state is seen by the viewer.

The biggest disadvantage of such a technique is that since each of thesub-pixels has a reflectance of about one third of the desired whitestate, the white state is fairly dim. To compensate this, a fourthsub-pixel may be added which can display only the black and whitestates, so that the white level is doubled at the expense of the red,green or blue color level (where each sub-pixel is only one fourth ofthe area of the pixel). Brighter colors can be achieved by adding lightfrom the white pixel, but this is achieved at the expense of color gamutto cause the colors to be very light and unsaturated. A similar resultcan be achieved by reducing the color saturation of the threesub-pixels. Even with this approach, the white level is normallysubstantially less than half of that of a black and white display,rendering it an unacceptable choice for display devices, such ase-readers or displays that need well readable black-white brightness andcontrast.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an electrophoretic display device of the presentinvention.

FIGS. 2(a)-2(c) illustrate three colors states of a display layer of thepresent invention.

FIGS. 3(a) and 3(b) shows two options where the display cells arealigned or unaligned, respectively, with the pixel electrodes.

SUMMARY OF THE INVENTION

The present invention not only provides a realistic solution for a colordisplay device which can display highly saturated color states, but alsoeliminates the need of color filters.

One aspect of the present invention is directed to a display layercomprising an electrophoretic medium and having a first surface on theviewing side and a second surface on the opposite side of the firstsurface, the electrophoretic medium which is sandwiched between a commonelectrode and a layer of pixel electrodes comprises a first type ofparticles, a second type of particles, a third type of particles and afourth type of particles which is additive particles, all dispersed in asolvent or solvent mixture, the first, second and third types ofparticles having respectively first, second and third opticalcharacteristics differing from one another, the first type of particleshaving a charge of one polarity and the second, third and fourth typesof particles having charges of the opposite polarity, and the secondtype of particles having an electric field threshold, such that:

(a) application of a voltage potential difference between the commonelectrode and a pixel electrode to generate an electric field strongerthan the electric field threshold and the voltage potential differencehaving the same polarity as the second type of particles, will cause apixel corresponding to the pixel electrode to display the second opticalcharacteristic at the first surface;

(b) application of a voltage potential difference between the commonelectrode and a pixel electrode to generate an electric field strongerthan the electric field threshold and the voltage potential differencehaving the same polarity as the first type of particles, will cause apixel corresponding to the pixel electrode to display the first opticalcharacteristic at the first surface; and

(c) once the first optical characteristic is displayed at the firstsurface, application of a voltage potential difference between thecommon electrode and a pixel electrode to generate an electric fieldweaker than the electric field threshold and the voltage potentialdifference having the same polarity as the third type of particles, willcause a pixel corresponding to the pixel electrode to display the thirdoptical characteristic at the first surface.

In one embodiment, the first type of particles and the second type ofparticles are of the white and black colors, respectively. In oneembodiment, the third type of particles is non-white and non-black. Inone embodiment, the third type of particles is of a color selected fromthe group consisting red, green and blue, magenta, yellow and cyan. Inone embodiment, the third type of particles is larger than the first orthe second type of particles. In one embodiment, the third type ofparticles is about 2 to about 50 times the sizes of the first or secondtype of particles. In one embodiment, the fourth type of particles iswhite. In one embodiment, the optical characteristic is color state.

In one embodiment, the electrophoretic medium is filled in display cellsand sandwiched between a common electrode and a layer of pixelelectrodes. In one embodiment, the display cells are cup-likemicrocells. In one embodiment, the display cells are microcapsules. Inone embodiment, the display cells are aligned with the pixel electrodes.In one embodiment, the display cells are not aligned with the pixelelectrodes. In one embodiment, the third type of particles is of thesame color in all display cells. In one embodiment, the third type ofparticles is of different colors in display cells.

In one embodiment, a pixel is driven by an electric field between thecommon electrode and a corresponding pixel electrode.

Another aspect of the present invention is directed to a driving methodfor a display layer comprising an electrophoretic medium and having afirst surface on the viewing side and a second surface on the oppositeside of the first surface, the electrophoretic medium which issandwiched between a common electrode and a layer of pixel electrodescomprises a first type of particles, a second type of particles, a thirdtype of particles and a fourth type of particles which is additiveparticles, all dispersed in a solvent or solvent mixture, the first,second and third types of particles having respectively first, secondand third optical characteristics differing from one another, the firsttype of particles having a charge of one polarity and the second, thirdand fourth types of particles having charges of the opposite polarity,and the second type of particles having an electric field threshold,which method comprises:

(a) applying a voltage potential difference between the common electrodeand a pixel electrode to generate an electric field stronger than theelectric field threshold and the voltage potential difference having thesame polarity as the second type of particles to cause a pixelcorresponding to the pixel electrode to display the second opticalcharacteristic at the first surface;

(b) applying a voltage potential difference between the common electrodeand a pixel electrode to generate an electric field stronger than theelectric field threshold and the voltage potential difference having thesame polarity as the first type of particles to cause a pixelcorresponding to the pixel electrode to display the first opticalcharacteristic at the first surface; and

(c) once the first optical characteristic is displayed at the firstsurface, applying a voltage potential difference between the commonelectrode and a pixel electrode to generate an electric field weakerthan the electric field threshold and the voltage potential differencehaving the same polarity as the third type of particles to cause a pixelcorresponding to the pixel electrode to display the third opticalcharacteristic at the first surface.

In one embodiment, in step (c), the voltage potential difference isapplied for not longer than 30 seconds. In one embodiment, in step (c),the voltage potential difference is applied for not longer than 15seconds.

Another aspect of the present invention is directed to a driving methodfor a display layer comprising an electrophoretic medium and having afirst surface on the viewing side and a second surface on the oppositeside of the first surface, the electrophoretic medium which issandwiched between a common electrode and a layer of pixel electrodescomprises a first type of particles, a second type of particles, a thirdtype of particles and a fourth type of particles which is additiveparticles, all dispersed in a solvent or solvent mixture, the first,second and third types of particles having respectively first, secondand third optical characteristics differing from one another, the firsttype of particles having a charge of one polarity and the second, thirdand fourth types of particles having charges of the opposite polarity,and the second type of particles having an electric field threshold,which method comprises driving a pixel from a color state of the firsttype of particles to a color state of the third type of particles byapplying an electric field which is weaker than the electric fieldthreshold of the second type of particles.

In one embodiment, when the color of the third type of particles is seenat a viewing side, the first and second types of particles gather at theside opposite of the viewing side resulting in an intermediate colorbetween the colors of the first and second types of particles.

DETAILED DESCRIPTION OF THE INVENTION

An electrophoretic fluid of present invention comprises four types ofparticles dispersed in a dielectric solvent or solvent mixture. For easeof illustration, the four types of particles may be referred to as afirst type of particles, a second type of particles, a third type ofparticles and a fourth type of particles. The fourth type of particlesis additive particles. The term “electrophoretic fluid” may also bereferred to as “electrophoretic medium”.

As an example shown in FIG. 1, the first type of particles is the whiteparticles (11), the second type of particles is the black particles(12), the third type of particles is the colored particles (13) and thefourth type of particles is the additive particles (14). The coloredparticles (13) are non-white and non-black particles.

It is understood that the scope of the invention broadly encompassesfluids comprising particles of any colors as long as among the fourtypes of particles, three types (i.e., the first type of particles, thesecond type of particles and the third type of particles) have visuallydistinguishable colors.

For white particles, they may be formed from an inorganic pigment, suchas TiO₂, ZrO₂, ZnO, Al₂O₃, Sb₂O₃, BaSO₄, PbSO₄ or the like.

For black particles, they may be formed from CI pigment black 26 or 28or the like (e.g., manganese ferrite black spinel or copper chromiteblack spinel) or carbon black.

The third type of particles may be of a color such as red, green, blue,magenta, cyan or yellow. The pigments for this type of particles mayinclude, but are not limited to, CI pigment PR 254, PR122, PR149, PG36,PG58, PG7, PB28, PB15:3, PY138, PY150, PY155 and PY20. These arecommonly used organic pigments described in color index handbooks, “NewPigment Application Technology” (CMC Publishing Co, Ltd, 1986) and“Printing Ink Technology” (CMC Publishing Co, Ltd, 1984). Specificexamples include Clariant Hostaperm Red D3G 70-EDS, Hostaperm PinkE-EDS, PV fast red D3G, Hostaperm red D3G 70, Hostaperm Blue B2G-EDS,Hostaperm Yellow H4G-EDS, Hostaperm Green GNX, BASF Irgazine red L 3630,Cinquasia Red L 4100 HD, and Irgazin Red L 3660 HD; Sun Chemicalphthalocyanine blue, phthalocyanine green, diarylide yellow or diarylideAAOT yellow.

In addition to the colors, the first, second and third types ofparticles may have other distinct optical characteristics, such asoptical transmission, reflectance, luminescence or, in the case ofdisplays intended for machine reading, pseudo-color in the sense of achange in reflectance of electromagnetic wavelengths outside the visiblerange.

The fourth type of particles (i.e., additive particles) has colorblocking and/or color enhancing properties, and therefore they may alsobe referred to as “color enhancing” particles. The additive particlesmay be of any color and they only serve to enhance the color of otherparticles. In other words, a display device utilizing a display fluid ofthe present invention does not display a color state of the fourth typeof particles (i.e., additive particles).

The additive particles are usually white. The white additive particlesmay be formed from an inorganic pigment, such as TiO₂, ZrO₂, ZnO, Al₂O₃,Sb₂O₃, BaSO₄, PbSO₄ or the like. These pigments are suitable becausethey have high refractive index and light scattering effect. In oneembodiment of the present invention, after surface treatment, theadditive particles will have similar or same charge polarity andmobility as the third type of particles (i.e., the colored particles).Therefore the additive particles will move together with, or veryclosely follow, the colored particles under an electric field. As aresult, the additive particles can help block the colors of the firsttype of particles and the second type of particles from being seen fromthe viewing side. This improves the hiding power of the third type ofparticles (i.e., the colored particles) and also enhances the brightnessof the color state displayed by the colored particles.

The additive particles are particularly useful when the coloredparticles are formed of an organic pigment which has relatively poorhiding power and coloring strength. An example of this would be yellowpigment PY154 which has weak hiding power, and when such a yellowpigment is used for the colored particles, the white additive particlescan provide the yellow pigment better hiding power and higherbrightness.

A display layer utilizing the display fluid of the present invention hastwo surfaces, a first surface (17) on the viewing side and a secondsurface (18) on the opposite side of the first surface (17). The displayfluid is sandwiched between the two surfaces. On the side of the firstsurface (17), there is a common electrode (15) which is a transparentelectrode layer (e.g., ITO), spreading over the entire top of thedisplay layer. On the side of the second surface (18), there is anelectrode layer (16) which comprises a plurality of pixel electrodes (16a).

The pixel electrodes are described in U.S. Pat. No. 7,046,228, thecontent of which is incorporated herein by reference in its entirety. Itis noted that while active matrix driving with a thin film transistor(TFT) backplane is mentioned for the layer of pixel electrodes, thescope of the present invention encompasses other types of electrodeaddressing as long as the electrodes serve the desired functions.

Each space between two dotted vertical lines in FIG. 1 denotes a pixel(10). As shown, each pixel has a corresponding pixel electrode. Anelectric field is created for a pixel by the potential differencebetween a voltage applied to the common electrode and a voltage appliedto the corresponding pixel electrode.

The percentages of the four types of particles in the fluid may vary. Asan example, in a fluid of black/white/colored/additive particles, theblack particle may take up about 0.1% to 10%, preferably 0.5% to 5%, byvolume of the electrophoretic fluid; the white particle may take upabout 1% to 50%, preferably 5% to 15%, by volume of the fluid; and thecolored particles may take up 2% to 20%, preferably 4% to 10%, by volumeof the fluid. The percentage of the additive particles (14) can be 0.1%to 5%, preferably 0.5% to 3%, by volume of the electrophoretic fluid.

The solvent in which the three types of particles are dispersed is clearand colorless. It preferably has a low viscosity and a dielectricconstant in the range of about 2 to about 30, preferably about 2 toabout 15 for high particle mobility. Examples of suitable dielectricsolvent include hydrocarbons such as isopar, decahydronaphthalene(DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oil, siliconfluids, aromatic hydrocarbons such as toluene, xylene,phenylxylylethane, dodecylbenzene or alkylnaphthalene, halogenatedsolvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene,dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluoride,chloropentafluoro-benzene, dichlorononane or pentachlorobenzene, andperfluorinated solvents such as FC-43, FC-70 or FC-5060 from 3M Company,St. Paul Minn., low molecular weight halogen containing polymers such aspoly(perfluoropropylene oxide) from TCI America, Portland, Oreg.,poly(chlorotrifluoro-ethylene) such as Halocarbon Oils from HalocarbonProduct Corp., River Edge, N.J., perfluoropolyalkylether such as Galdenfrom Ausimont or Krytox Oils and Greases K-Fluid Series from DuPont,Del., polydimethylsiloxane based silicone oil from Dow-corning (DC-200).

The first and second types of particles carry opposite chargepolarities. The third and fourth types of particles have the same chargepolarity as one of the first and second types of particles. In a fluidof black/white/colored/additive particles, if the black particles arepositively charged and the white particles are negatively charged, thenboth the colored and additive particles may be either positively chargedor negatively charged.

In addition, the charges carried by the colored and additive may beweaker than the charges carried by the black and white particles. Theterm “weaker charge” is intended to refer to the charge of the particlesbeing less than about 50%, preferably about 5% to about 30%, of thecharge of the stronger charged particles.

The charge potentials of the particles may be measured in terms of zetapotential. In one embodiment, the zeta potential is determined byColloidal Dynamics AcoustoSizer IIM with a CSPU-100 signal processingunit, ESA EN# Attn flow through cell (K:127). The instrument constants,such as density of the solvent used in the sample, dielectric constantof the solvent, speed of sound in the solvent, viscosity of the solvent,all of which at the testing temperature (25° C.) are entered beforetesting. Pigment samples are dispersed in the solvent (which is usuallya hydrocarbon fluid having less than 12 carbon atoms), and diluted to be5-10% by weight. The sample also contains a charge control agent(SOLSPERSE® 17000, available from Lubrizol Corporation, a BerkshireHathaway company), with a weight ratio of 1:10 of the charge controlagent to the particles. The mass of the diluted sample is determined andthe sample is then loaded into the flow-through cell for determinationof the zeta potential.

The four types of particles may also have varying sizes. In oneembodiment, one or two types of the four types of particles may belarger than the other types. It is noted that among the four types ofparticles, the colored particles preferably have a larger size. Forexample, both the black and white particles are relatively small andtheir sizes (tested through dynamic light scattering) may range fromabout 50 nm to about 800 nm and more preferably from about 200 nm toabout 700 nm, and the colored particles which have a weaker charge,preferably are about 2 to about 50 times and more preferably about 2 toabout 10 times the average sizes of the black or white particles. Thefourth type of particles (i.e., additive particles) may be of any size.

In the present invention, at least one type of particles may demonstratean electric field threshold. In one embodiment, one type of the highercharged particles has an electric field threshold.

The term “electric field threshold”, in the context of the presentinvention, is defined as the maximum electric field that may be appliedfor a period of time (typically not longer than 30 seconds, preferablynot longer than 15 seconds), to a group of particles, without causingthe particles to appear at the viewing side of a pixel, when the pixelis driven from a color state different from the color state of the groupof particles. The term “viewing side”, in the present application,refers to the first surface in a display layer where images are seen bythe viewers.

The electric field threshold is either an inherent characteristic of thecharged particles or an additive-induced property.

In the former case, the electric field threshold is generated, relyingon certain attraction force between oppositely charged particles orbetween particles and certain substrate surfaces.

In the case of additive-induced electric field threshold, a thresholdagent which induces or enhances the threshold characteristics of anelectrophoretic fluid may be added. The threshold agent may be anymaterial which is soluble or dispersible in the solvent or solventmixture of the electrophoretic fluid and carries or induces a chargeopposite to that of the charged particles. The threshold agent may besensitive or insensitive to the change of applied voltage. The term“threshold agent” may broadly include dyes or pigments, electrolytes orpolyelectrolytes, polymers, oligomers, surfactants, charge controllingagents and the like.

Additional information relating to the threshold agent may be found inU.S. Pat. No. 8,115,729, the content of which is incorporated herein byreference in its entirety.

The following is an example illustrating the present invention.

EXAMPLE

This example is demonstrated in FIG. 2. The black particles (22) areassumed to have an electric field threshold. Therefore, the blackparticles (22) would not move to the viewing side if an applied electricfield is weaker than the electric field threshold.

The white particles (21) are negatively charged while the blackparticles (22) are positively charged, and both types of particles aresmaller than the colored particles (23). It is assumed, for illustrationpurpose, that the colored particles (23) are of the yellow color and theadditive particles (24) are of the white color.

The yellow particles (23) and the white additive particles (24) carrythe same charge polarity as the black particles which have the electricfield threshold, but they carry a weaker charge than the blackparticles. As a result, the black particles move faster than the yellowparticles (23) and the white additive particles (24) because of thestronger charge carried by the black particles, when an applied electricfield is greater than the electric field threshold of the blackparticles.

In FIG. 2a , the applied voltage potential difference is +15V. In thiscase, an electric field generated by the applied driving voltage isgreater than the electric field threshold, and therefore it causes thewhite particles (21) to move to be near or at the pixel electrode (26)and the black particles (22), the yellow particles (23) and the whiteadditive particles (24) to move to be near or at the common electrode(25). As a result, a black color is seen at the viewing side. The yellowparticles (23) and the white additive particles (24) move towards thecommon electrode (25); however because they carry weaker charges, theymove slower than the black particles.

In FIG. 2b , when a voltage potential difference of −15V is applied. Inthis case, an electric field generated has an opposite polarity and itis also greater than the electric field threshold. As a result, itcauses the white particles (21) to move to be near or at the commonelectrode (25) and the black particles (22), the yellow particles (23)and the white additive particles (24) to move to be near or at the pixelelectrode (26). Consequently, a white color is seen at the viewing side.

The yellow particles (23) and the white additive particles (24) movetowards the pixel electrode because they are also positively charged.However, because they carry weaker charges, they move slower than theblack particles.

In FIG. 2c , the applied voltage potential difference changes to +5V. Inthis case, an electric field generated is weaker than the electric fieldthreshold and therefore it causes the negatively charged white particles(21) in FIG. 2(b) to move towards the pixel electrode (26). The blackparticles (22) move little because of their electric field threshold.Due to the fact that the yellow particles (23) and the white additiveparticles (24) do not have a significant electric field threshold, theymove to be near or at the common electrode (25) and as a result, thecolor of the yellow particles is seen at the viewing side and the whiteadditive particles (24) block the black and white particles from beingseen at the viewing side, thus enhancing the yellow color state.

Also as shown in FIG. 2(c), when the yellow particles and the whiteadditive particles are at the common electrode side (i.e., the viewingside), the black and white particles are mixed at the non-viewing side,forming an intermediate color state (i.e., grey) between the white andblack particles.

The electrophoretic fluid in an electrophoretic display device is filledin display cells. The display cells may be cup-like microcells asdescribed in U.S. Pat. No. 6,930,818, the content of which isincorporated herein by reference in its entirety. The display cells mayalso be other types of micro-containers, such as microcapsules,microchannels or equivalents, regardless of their shapes or sizes. Allof these are within the scope of the present application.

In one embodiment of the present invention, the display device utilizingthe present electrophoretic fluid is a high-light display device and inthis embodiment, the colored particles are of the same color in alldisplay cells. In this case, as shown in FIG. 3, the display cells (31)may be aligned with the pixel electrodes (32) (see FIG. 3a ) orun-aligned with the pixel electrodes (see FIG. 3b ).

In another embodiment, the display device utilizing the presentelectrophoretic fluid may be a multi-color display device. In thisembodiment, the colored particles are of different colors in the displaycells. In this embodiment, the display cells and the pixel electrodesare aligned.

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the nd scope of the invention. Inaddition, many modifications may be made to adapt a particularsituation, materials, compositions, processes, process step or steps, tothe objective, spirit and scope of the present invention. All suchmodifications are intended to be within the scope of the claims appendedhereto.

What is claimed is:
 1. A display layer having a first, viewing surfaceand a second surface on the opposite side of the display layer from thefirst surface; the display layer further comprising an electrophoreticmedium and means for applying an electric field between the first andsecond surfaces, the electrophoretic medium comprising a fluid andfirst, second, third and fourth types of particles dispersed in thefluid, the fourth type of particles being additive particles, the first,second and third types of particles having respectively first, secondand third optical characteristics differing from one another; the firsttype of particles having a charge of one polarity and the second, thirdand fourth types of particles having charges of the opposite polarity,and the second type of particles having an electric field threshold;such that: (a) application between the first and second surfaces of anelectric field stronger than the electric field threshold and a polaritydriving the second type of particles towards the viewing surface, willdisplay the second optical characteristic at the viewing surface; (b)application between the first and second surfaces of an electric fieldstronger than the electric field threshold and a polarity driving thefirst type of particles towards the viewing surface, will display thefirst optical characteristic at the viewing surface; and (c) once thefirst optical characteristic is displayed at the viewing surface,application between the first and second surfaces of an electric fieldweaker than the electric field threshold and having a polarity drivingthe third type of particles towards the viewing surface, will displaythe third optical characteristic at the viewing surface.
 2. The displaylayer of claim 1, wherein the first and second optical characteristicsare white and black respectively.
 3. The display layer of claim 1,wherein the third optical characteristic is non-white and non-black. 4.The display layer of claim 3, wherein the third optical characteristicis selected from the group consisting red, green and blue, magenta,yellow and cyan.
 5. The display layer of claim 1, wherein the third typeof particles is larger than the first or the second type of particles.6. The display layer of claim 5, wherein the third type of particles isabout 2 to about 50 times the size of the first or second type ofparticles.
 7. The display layer of claim 1, wherein the fourth type ofparticles is white.
 8. The display layer of claim 1, wherein theelectrophoretic medium is filled in display cells.
 9. The display layerof claim 8, wherein the display cells are microcells.
 10. The displaylayer of claim 8, wherein the display cells are microcapsules.
 11. Thedisplay layer of claim 8, wherein the display cells are aligned with thepixel electrodes.
 12. The display layer of claim 8, wherein the displaycells are not aligned with the pixel electrodes.
 13. The display layerof claim 8 wherein the third type of particles is of the same color inall display cells.
 14. The display layer of claim 8, wherein the thirdtype of particles is of different colors in display cells.
 15. Thedisplay layer of claim 1, wherein in step (c), the electric field isapplied for not longer than 30 seconds.
 16. The display layer of claim15, wherein in step (c), the electric field is applied for not longerthan 15 seconds.